15 P Phosphorus

Click image to magnify. Two pieces of ultrapure purple phosphorus. Original size in cm: 0.5 x 2
Phosphorus is a very common element, which is found in every life form, notably as the complex molecule adenosine triphosphate (ATP), which supplies the cells with energy. As an element it has four different allotropes, white, red, black and purple. The white phosphorus is infamous for its extreme toxicity and dangerousness, it spontaneously burns in air. The other allotropes are more or less harmless. Phosphates are a main ingredient of fertilizers and as such are often a big ecological problem for waterbodies.

Left: White phosphorus, containing a small amount of red phosphorus, which gives it a yellow color. 0.5 x 1 cm, about 1 gram.
Right: two and a half years later the phosphorus turned perceptibly redder.